The present invention relates to filter circuits, and more particularly, to filters including acoustic resonators used as filters for electronic circuits.
The need to reduce the cost and size of electronic equipment has led to a continuing need smaller filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Hence, there has been a continuing effort to provide inexpensive, compact filter units.
One class of filters that has the potential for meeting these needs is constructed using a thin film bulk acoustic resonators (FBARs). These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In a typical configuration as illustrated by FIG. 1, a filter 10 includes a series resonator 12 connected to a shunt inductor 14 which may be adjustable for the purposes of tuning the filter 10.
The resonator 12 is fabricated on a semiconductor substrate that is packaged as a chip and mounted on a printed circuit board (PCB). Typically, the inductor 14 is attached to the PCB or mounted in a module using multiple chips and wire bonds, separate from the chip containing the resonator 10. Then, the inductor is connected to the resonator via output connector leads from the chip including the resonator 10.
Such external inductor connections lead to signal loss between the resonator 10 and the inductor 14 and decrease in reliability due to the connection. Also, the manufacturing step must involve soldering or a step of attaching the resonator 12 on the chip to the inductor 14 located outside the chip.
Consequently, there is a need to provide an FBAR filter overcoming these shortcomings.
These needs are met by the present invention. According to one aspect of the present invention, an apparatus having a resonator and an inductor, both fabricated on a single substrate is disclosed. The resonator is fabricated bridging a first cavity; and the inductor is fabricated bridging a second cavity. The inductor is connected to the resonator.
According to a second aspect of the present invention, an electronic filter including a resonator is fabricated on a substrate bridging a first cavity and an inductor is fabricated on the substrate bridging a second cavity, the inductor connected to the resonator.
According to a third aspect of the present invention, a method of fabricating an electronic filter on a substrate is disclosed. First, a resonator is fabricated on the substrate. In addition, an inductor, connected to the resonator, is fabricated on the substrate.
According to a fourth aspect of the present invention, another method of fabricating an electronic filter on a substrate is disclosed. First, a cavity is fabricated within the substrate. Next, the cavity is filled with sacrificial material. Then, an inductor is fabricated above the cavity. Finally, the sacrificial material is removed from the cavity.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in combination with the accompanying drawings, illustrating by way of example the principles of the invention.